The effect of simulated acid rain on net photosynthesis of two common boreal forest lichens, Cladina stellaris and Cladina rangiferina, was investigated. Repeated measurements of net photosynthetic rate (NPR) were conducted on podetial tips over a week-long period to assess both immediate response to an acid spray treatment (2:1 H2SO4:HNO3; pH range 2.5 to 3.5) and subsequent photosynthetic recovery after the solution of artificial rain had been rinsed out of the cells. A control group, which was treated with pH 5.6 episodes, was used to monitor natural fluctuations in NPR over time in a laboratory environment. Although lowered photosynthetic rates were observed for both species immediately following a low pH treatment, differences between groups were not significant. Three days following treatment, however, significant decreases in photosynthesis of up to 40% of pretreatment values were found for pH 2.5 and 3.0-sprayed C. stellaris, and a decline in NPR of 25% was recorded for pH 3.0-treated C. rangiferina. Six days after the simulated rain episode, a 20% recovery of photosynthesis had occurred in pH 2.5-treated C. stellaris, while pH 3-0-sprayed C. rangiferina underwent a 20% stimulation of NPR compared to the pretreatment value. In spite of the demonstrated ability to recover partially following a single low pH event, it is hypothesized that even temporary declines in NPR could restrict growth potential and biomass accumulation of boreal cyptogamic species which have a short growing season and photosynthesize only when moisture is available. Results of the study indicate that C. stellaris is more sensitive to single low pH events than is C. rangiferina.